A cold-rolling process has been known as a production process of a commercially pure titanium sheet, the process being capable of providing a product with high accuracy that inhibits surface oxidization and thermal contraction. The cold-rolling process typically comprises a hot-rolling step, an annealing step, a first cold-rolling step, an intermediate annealing step, a second cold-rolling step, and a finishing annealing step. In the cold-rolling process: a rolled body is formed by the hot-rolling step and the annealing step; in the first cold-rolling step, the rolled body is rolled to form a rolled sheet having an intermediate thickness; and the rolled sheet is subjected to the intermediate annealing step and then again to the cold-rolling (second cold-rolling step), whereby a rolled sheet having a product thickness is formed. In other words, in the cold-rolling process, the cold-rolling step and the annealing step are repeated to gradually thin the rolled sheet. Furthermore, in such a cold-rolling process, a total rolling reduction (rolling reduction being a percentage reduction in thickness after rolling) is small in each of the first cold-rolling step and the second cold-rolling step in order to prevent a crack and a rupture of the rolled sheet.
On the other hand, in these days, an increasing demand for commercially pure titanium sheets and an increasing emphasis on reduction of production cost, etc. have been accompanied by a need for omission of steps.
With regard to the omission of steps, for example, omission of the first cold-rolling step and the intermediate annealing step is under study. However, omitting the first cold-rolling step and the intermediate annealing step requires a large increase in the total rolling reduction in the second cold-rolling step. Consequently, such a conventional process is unable to maintain sufficient ductility of the rolled sheet during the cold-rolling in the second cold-rolling step, whereby an edge crack and a rupture of the rolled sheet may be generated during the cold-rolling.
In this regard, as a measure for preventing such an edge crack and a rupture, the thickness of the rolled sheet formed by the hot-rolling step may be reduced to decrease the total rolling reduction in the second cold-rolling step. However, such a measure may cause a rolling failure in the hot-rolling step and may lower a pickling yield in the annealing step following the hot-rolling step.
A technique of omitting steps alternative to the omission of the first cold-rolling step and the intermediate annealing step (technique of omitting steps other than the first cold-rolling step and the intermediate annealing step) is exemplified by techniques disclosed in Patent Documents 1 and 2.
Patent Document 1 discloses a production method of a high-strength pure titanium sheet comprising hot-rolling in one direction at a heating temperature of greater than or equal to 840° C. and less than 920° C. to attain a rolling reduction of greater than or equal to 95%, without being followed by the annealing.
Patent Document 2 discloses a production method of a hot-rolled titanium sheet comprising: forcedly cooling a hot-rolled titanium sheet; and then coiling the sheet at a temperature of less than or equal to 500° C. It is disclosed that the method enables the hot-rolled titanium sheet to serve as a material for cold-rolling as is, without annealing. In other words, in the production method of a titanium sheet proposed in Patent Document 2, the annealing step between the hot-rolling step and the first cold-rolling step can be omitted. Furthermore, the production method enables a product quality to be uniform without the annealing step, due to subjecting the hot-rolled titanium sheet to quenching and then to the coiling at the temperature not causing grain growth.